The present invention relates to a device and a method for detecting a focusing state of an objective by receiving light beams emitted from an object and passing through two portions of the objective that are disposed oppositely with respect to an optical axis of the objective.
Japanese Patent Application Laying Open (KOKAI) No.62-102213 discloses a focus detecting device. In the focus detecting device disclosed, a charge coupled device (CCD) receives first and second images as are formed by the light beams emitted from an object and passing through first and second portions of an objective or photographic lens that are oppositely disposed with respect to an optical axis of the objective. The focus detecting device calculates an integer part and a fractional part of a displacement of the first and second images one from the other by using the output signals of the CCD, and detects a displacement of the objective from its focal position.
In the integer part calculation, a distance between the first and the second images is worked out roughly or with relatively large pitches. In the fractional part calculation, the distance obtained by the integer part calculation is worked out precisely or with small pitches. The integer part calculation provides the interimage distance with relatively large pitches. Therefore, the resultant interimage distance will determine also the direction of the movement of the lens, and greatly influences a view through the finder. Accordingly, it is capable of reliably detecting the interimage distance of even an object whose distance is hard to measure. The integer part calculation being excellent in detecting the interimage distance sometimes loses its reliability. Such a case results from poor or no contrast of an object to be photographed. When an object of no contrast exists within a view of a finder, the focus detecting device is unable to measure a distance of the object, and repeats only the integration of the CCD and the integer part calculation. In such a situation, if an object of a high contrast suddenly enters the view of the finder, the focus detecting device operates to obtain an optimum focus through the ordinary distance measurement and lens drive.
There has been known another focus detecting device, called a single focus detecting device. In the above situation, viz., the object has no contrast and its distance cannot be measured, the single focus detecting device, unlike the focus detecting device of the gazette, will not operate until the release button is pushed again. Accordingly, an operator sometimes fails to shoot the object at good timing. Where an objective of a long focal distance is used, tremulous hands make it difficult to place an object within a measurable range. Further, when the object is a thing moving at a high speed, a high skill is required to trace the object within the measurable range.